1. Field of the Invention
This invention relates to a power supply apparatus and a power supply control device, and particularly to a power supply apparatus and a power supply control device to reduce undershoot of source voltage when a power supply interrupt occurs.
2. Background of the Invention
Conventional integrated circuit power supply systems include, a DC/DC converter with a synchronous commutation type switching regulator. When a power supply incorporating a DC/DC converter experiences an interrupt, a synchronous commutation FET is commonly used to discharge an output capacitor on the low voltage side of the DC/DC converter.
However, this approach suffers from a rapid decrease in source voltage as the output capacitor discharges, which may result in voltage undershoot of the power supply. Japanese Published Unexamined Patent Application (PUPA) 10-295074 (pp. 3 and 6, FIG. 2) discloses a method of preventing undershoot whereby a synchronous commutation FET coupled to the low voltage side of a DC/DC converter is turned off. At the onset of a power supply interrupt, an output capacitor is gradually discharged through a resistance connected between an output terminal of the source voltage and ground, which divides the voltage from the output terminal.
An aluminum or tantalum output capacitor is disclosed in Japanese PUPA 10-295074, which exhibits a dedicated polarity for each terminal and cannot be reverse biased without damaging the device. Therefore, discharging an output capacitor with an active synchronous commutation FET on the low voltage side of the DC/DC converter may result in a reverse bias of the output capacitor due to the voltage undershoot caused by the power supply interrupt, leading in turn to mechanical failure of the output capacitor. This is not an unusual occurrence since semiconductor devices connected to a power supply apparatus typically specify an allowable reverse voltage value. Accordingly, if reverse voltage with undershoot is applied to such a semiconductor device, malfunction or physical damage may result.
Conversely, if an output capacitor is gradually discharged while a synchronous commutation FET on the low voltage side of the DC/DC converter is turned off, the source voltage slowly decreases and a reverse voltage outside the specification of the semiconductor may be applied. In this instance, the probability of malfunction of the semiconductor is increased.